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MILLER’S 

White  Cloth  Flap  Binders, 

MANUFACTURED  BY 

HAROLD  E.  MILLER 

540-542-544-546  B’way,  Albany,  M.  Y. 

SEND  FOR  PRICES. 

A COMPARATIVE  STUDY 


THE  METHODS  USED 
FOR  THE  MEASUREMENT 


OF  THE 


TURBIDITY  OF  WATER 


GEORGE  C.  WHIPPLE 
DANIEL  D.  JACKSON 


Reprinted  from  Technology  Quarterly,  Vol.  XIII,  No,  3,  September,  1900 


UNIVERSITY  OF  ILLINOIS 


CHEMISTRY  DEPARTMENT 


ARTHUR  WILLIAM  PALMER 
MEMORIAL  LIBRARY 
1904 


W51 


ytU  \)  L.  V\ 


274 


George  C.  Whipple  and  Daniel  D.  Jackson. 


IT 

£ 


A COMPARATIVE  STUDY  OF  THE  METHODS  USED 
FOR  THE  MEASUREMENT  OF  THE  TURBIDITY  OF 
WA  TER. 

By  GEORGE  C.  WHIPPLE  and  DANIEL  D.  JACKSON. 

Received  July  25,  1900. 

At  the  present  time,  several  different  methods  for  the  measure- 
ment of  the  amount  of  suspended  matter  in  water  are  in  common 
use.  The  most  important  of  these  are  : i.  The  Gravimetric  Method. 
2.  The  Wire  Method.  3.  The  Diaphanometer  Method.  4.  The 
use  of  Standards  of  Comparison.  In  addition  to  these  are  the  Disc 
Method  and  the  Photometer  Method,  the  former  applicable  only  to 
the  comparatively  clear  water  of  lakes  and  reservoirs,  and  the  latter 
too  complicated  for  ordinary  use. 

The  weight  of  the  suspended  matter  present  in  water  is  found  by 
taking  the  difference  between  the  total  solids  before  and  after  filtra- 
tion through  filter  paper  or  through  a Pasteur  filter. 

The  wire  method,  brought  into  use  by  Hazen,1  consists  in  the 
observation  of  the  visibility  of  a platinum  wire  lowered  horizontally 
into  the  water.  The  turbidity  scale  is  furnished  by  the  reciprocal  of 
the  depth  in  inches  at  which  the  wire  becomes  invisible. 

The  diaphanometer  is  an  instrument  devised  by  Hornung  and 
improved  by  Parmelee  and  Ellms,2  according  to  which  the  turbidity 
is  measured  by  noting  the  limiting  depth  of  liquid  in  a tube  through 
which  an  image  at  the  bottom  can  be  discerned  under  standard  con- 
ditions of  illumination.  The  reciprocal  scale  is  used  as  in  the  case 
of  the  wire*  method. 

The  use  of  kaolin  standards  of  comparison  was  suggested  by 
Mason,3  but  the  authors  have  found  that  finely  divided  silica,  obtained 
from  diatomaceous  earth,  is  more  satisfactory.  According  to  these 

1 Hazen,  Allen.  The  Filtration  of  Public  Water  Supplies.  3d  edition.  New  York: 
Wiley. 

2 Parmelee  and  Ellms.  On  Rapid  Methods  for  the  Estimation  of  the  Weight  of  Sus 
pended  Matter  in  Turbid  Waters.  Technology  Quarterly , Vol.  xii,  No.  2,  June,  1899. 

3 Mason,  Wm.  P.  Examination  of  Water.  New  York:  Wiley. 


4!  * > o o 


Methods  Used  for  Measurement  of  Turbidity  of  Water.  275 

methods  the  turbidity  of  the  water  is  estimated  by  comparing  it  with 
a series  of  standards  of  varying  turbidity  in  glass  tubes,  the  stand- 
ards being  prepared  by  adding  definite  amounts  of  silica  to  distilled 
water,  and  the  results  expressed  in  parts  per  million  of  silica.1 

These  four  methods  are  all  fairly  satisfactory,  but  each  one  has  its 
objections  and  its  limitations.  The  wire  method  is  the  simplest  and 
in  many  cases  the  most  practical,  but  its  results  are  only  approxi- 
mately correct,  and  its  use  is  limited  to  the  hours  of  bright  daylight. 
The  gravimetric  method  is  a long  process,  and  requires  careful 
manipulation  and  accurate  weighing.  It  takes  no  account  of  the 

state  of  division  of  the  suspended  matter.  The  diaphanometer 
method  is  of  more  general  applicability,  but  it  demands  a somewhat 
elaborate  apparatus  and  a uniform  source  of  light.  The  use  of  silica 
standards  of  comparison  is  satisfactory  .for  waters  of  low  turbidity, 
but  less  so  when  the  turbidity  is  very  high.  The  comparisons  cannot 
be  well  made  with  artificial  light. 

Each  of  these  methods,  therefore,  has  its  especial  field  of  applica- 
bility. The  wire  method  is  best  adapted  to  field  work  and  to  the 
study  of  the  turbidity  of  streams  where  a single  daily  observation  is 
sufficient ; the  diaphanometer  is  of  use  in  connection  with  the  opera- 
tion of  filters,  where  it  is  necessary  to  continue  the  observations 
through  the  night ; and  the  method  of  comparison  with  standards  is 
suitable  for  general  laboratory  use.  It  is  neither  practical  nor  desir- 
able, therefore,  to  limit  in  any  way  the  use  of  these  various  methods, 
but  it  is  desirable  that  the  relations  that  exist  between  the  different 
methods  be  definitely  known.  The  great  importance  of  the  knowl- 
edge of  the  turbidity  of  our  American  streams  and  the  magnitude  of 
the  field  of  observation  render  it  imperative  that  some  standard  of 
turbidity  shall  be  universally  recognized.  Only  thus  can  the  results 
obtained  by  the  different  methods  be  made  comparable. 

Such  a standard  must  be  one  that  is  permanent  and  capable  of 
exact  duplication  by  different  persons,  and  it  must  be  of  such  a nature 
that  comparisons  may  be  readily  made  with  any  of  the  methods  now 
in  use.  The  scale  of  turbidity  must  be  a uniform  one,  that  is,  the 
figures  that  express  the  turbidity  must  be  directly  proportional  to 
the  amount  of  suspended  matter  present  in  any  given  state  of  sub' 
division. 


1 Whipple,  G.  C.,  and  Jackson,  D.  D.  Silica  Standards  for  the  Determination  of  the 
Turbidity  of  Water.  Technology  Quarterly,  Vol.  xii,  No.  4,  December,  1S99. 


276 


George  C.  Whipple  and  Daniel  D.  Jackson. 


With  this  object  in  view,  the  authors  have  carefully  studied  the 
methods  now  in  use  and  the  relations  that  exist  between  them,  and 
have  reached  the  following  conclusions  : 

1.  No  optical  method  based  upon  the  reciprocal  scale  can  serve 
as  a standard,  for  such  a scale  is  not  a uniform  one. 

2.  In  order  to  obtain  a uniform  scale  with  an  optical  method,  it 
is  necessary  to  calibrate  the  apparatus  used,  to  correspond  with  frac- 
tional dilutions  of  a water  of  definite  turbidity. 

3.  The  water  of  definite  turbidity  thus  used  should  be  considered 
as  the  standard  of  turbidity,  and  turbidity  readings  by  all  methods 
should  be  expressed  in  terms  of  this  standard. 

4.  Such  a definite  standard  of  turbidity  is  found  in  the  use  of 
finely  ground  diatomaceous  earth,  as  described  by  the  authors.1 

The  Gravimetric  Method. 

The  determination  of  the  weight  of  suspended  matter  in  water  is 
not  an  exact  process.  For  various  reasons  the  results  are  liable  to  be 
in  error  by  5 parts  per  million,  and  at  times  by  more  than  this  amount. 
With  very  turbid  waters  the  percentage  of  error,  however,  is  small,  but 
when  the  water  contains  less  than  25  parts  per  million  of  suspended 
matter  the  error  is  considerable,  and  when  below  5 parts  per  million 
the  results  are  practically  worthless.  If  the  particles  of  suspended 
matter  are  large,  they  can  be  separated  from  the  water  by  using  a 
close  filter-paper ; but  when  the  water  contains  fine  silt,  clay,  etc.,  it 
is  necessary  to  use  a Pasteur  tube. 

The  Wire  Method. 

The  accurate  observation  of  turbidity  by  the  wire  method  demands 
constant  conditions  of  light  and  a wire  of  uniform  size  and  brightness. 
Hazen  has  stated  that  the  observation  should  be  made  in  the  open  air 
during  the  middle  part  of  the  day,  and  with  the  wire  shaded  from 
direct  sunlight.  When  the  turbidity  of  the  water  is  such  that  the 
wire  can  be  seen  at  depths  greater  than  about  one  foot,  these  condi- 
tions are  necessary,  but  when  the  turbidity  is  greater  than  that  men- 
tioned, the  error  from  variations  in  light  appears  to  be  less.  Thus 
the  authors  have  found  that  when  the  turbidities  were  greater  than 
.20  on  the  reciprocal  scale,  the  same  readings  were  obtained  at  all 


Loc.  cit. 


Methods  Used  for  Measurement  of  Turbidity  of  Water.  277 

times  during  the  day  when  the  sun  was  more  than  about  io°  above 
the  horizon ; that  readings  made  under  a porch  agreed  with  those  in 
the  open  air,  and  differed  but  little  from  those  made  indoors  near  a 
large  window.  Direct  sunlight  introduces  an  uncertainty  in  the  read- 
ings, and  is  to  be  avoided.  The  nature  of  the  receptacle  containing 
the  turbid  water  has  practically  no  influence  on  the  reading,  provided 
that  the  diameter  is  at  least  twice  the  depth  at  which  the  wire 
becomes  invisible.  With  receptacles  of  smaller  diameter  than  this 
the  opacity  of  the  sides  has  a slight  effect  on  the  reading.  Accord- 
ingly, a water  pail  12  inches  in  diameter  should  not  be  used  for 
reading  turbidities  below  .17  on  the  reciprocal  scale.  It  is  prefer- 
able to  have  all  the  illumination  from  the  top  and  not  to  use  a glass 
jar  so  supported  that  light  may  enter  from  beneath.  Differences  in 
reading  may  be  obtained  by  changing  the  distance  of  the  eye  from 
the  water,  as  will  be  seen  from  the  following  table  : 


Distance  of  eye  above 
surface  of  water. 

Turbidity 

Readings. 

No.  1. 

No.  2. 

No.  3. 

No.  4. 

4 inches. 

00 

.50 

.26 

.145 

8 inches. 

.71 

.45 

.25 

.140 

12  inches. 

.73 

.47 

.26 

.140 

20  inches. 

.78 

.48 

.28 

.145 

50  inches. 

.82 

.49 

.29 

.150 

As  might  be  expected,  these  differences  are  greatest  in  the  case  of 
very  turbid  waters.  The  readings  are  most  accurate  when  the  distance 
of  the  wire  from  the  eye  is  about  equal  to  that  of  normal  vision. 
Throughout  the  present  investigations  this  distance  has  been  pre- 
served as  nearly  as  possible.  In  field  work,  however,  it  is  a com- 
mon practice  to  read  the  rod  in  a standing  position,  the  wire  thus 
being  from  four  to  six  feet  from  the  eye. 

The  size  of  the  platinum  wire  used  has  a slight  effect  on  the 
results.  At  Pittsburg  it  was  found  that  a wire  0.4  mm.  in  diameter 
gave  readings  25  per  cent,  higher  than  those  obtained  with  the  stand- 
ard wire,  1 mm.  in  diameter,  and  that  a wire  larger  than  the  standard 
gave  somewhat  lower  results.  The  authors  have  compared  turbidities 


2yS 


George  C.  Whipple  and  Daniel  D.  Jackson . 


with  wires  i mm.  and  1.5  mm.  in  diameters,  and  have  obtained  results 
that  were  in  substantial  agreement  with  each  other. 

Comparisons  between  the  wire  method  and  the  disc  method  1 have 
shown  that  the  latter  gives  somewhat  lower  results,  — that  is,  the 
disc  can  be  seen  at  greater  depths  than  the  wire.  This  is  shown  by 
the  following  table  : 


Turbidity  by  wire  method. 

Turbidity  by  disc  method. 

Per  cent,  which  the  turbidity  by  disc 
method  was  of  the  turbidity  by 
wire  method. 

.098 

.087 

90 

.120 

.105 

87 

.135 

.125 

91 

.183 

.165 

92 

.450 

.363 

80 

.609 

.444 

73 

.917 

.625 

68 

With  high  turbidities  the  differences  are  considerable,  but  with  com- 
paratively clear  water  the  results  of  the  two  methods  approach  each 
other.  Turbidity  reading  by  the  wire  method  may  be  therefore 
extended  into  the  clear  water  of  lakes  by  the  substitution  of  the  disc, 
without  the  introduction  of  serious  error. 

It  has  been  generally  assumed  that  the  turbidity  of  water  is  in- 
versely proportional  to  the  depth  at  which  the  wire  becomes  invisible, 
and  the  reciprocal  scale  is  based  upon  this  assumption.  Hazen  states 
“that  if  a water  having  a turbidity  of  1.00  is  mixed  with  an  equal 
volume  of  clear  water,  the  mixture  will  have  a turbidity  of  0.50,  and 
advantage  is  taken  of  this  fact  for  the  measurement  of  high  turbidities 
by  dilution.”  Upon  this  assumption  also  depends  the  accuracy  of  the 
elaborate  apparatus  for  turbidity  measurement  used  in  the  filtration 
experiments  at  Washington,2  D.  C.,  where  high  turbidities  were  read 
by  dilution  with  clear  water  and  where  low  turbidities  were  read  by 
admixture  with  a water  of  known  turbidity. 

1 Whipple,  G.  C.  The  Microscopy  of  Drinking  Water,  p.  73.  New  York  : Wiley. 

2 Report  of  Colonel  A.  M.  Miller,  on  the  Feasibility  and  Propriety  of  Filtering  the  Water 
Supply  of  Washington,  D.  C.  Senate  Document , No.  259,  56th  Congress,  1st  session. 


Methods  Used  for  Measurement  of  Turbidity  of  Water.  279 

According  to  the  experience  of  the  authors,  the  assumption  that 
the  turbidity  is  inversely  proportional  to  the  depth  of  the  disappearing 
wire,  is  only  approximately  correct  when  the  distance  of  the  wire 
from  the  eye  is  substantially  that  of  normal  vision.1  If  a water  in 
which  the  disappearing  wire  has  a depth  of  one  inch  is  diluted  with 
an  equal  volume  of  clear  water,  the  wire  will  not  disappear  at  a depth 
of  exactly  two  inches,  but  at  a depth  somewhat  less  than  two  inches  ; 
or,  in  other  words,  a turbid  water,  when  diluted  with  an  equal  vol- 
ume of  clear  water,  will  have  a turbidity,  according  to  the  reciprocal 
scale,  of  somewhat  more  than  one-half  the  original  turbidity.  This 
has  been  shown  by  scores  of  observations,  but  a single  example  will 
serve  as  an  illustration. 


Turbidity  calculated  according 
to  the  reciprocal  scale. 

Observed  turbidity. 

Original  water 

1.60 

1.60 

Diluted  ^ with  clear  water  . 

.80 

.83 

Diluted  ^ with  clear  water  . 

.40 

.47 

Diluted  ^ with  clear  water  . 

.20 

.26 

Diluted  j-q  with  clear  water  . 

.10 

.145 

This  fact  has  been  noticed  by  other  observers,  but  its  important 
bearing  upon  the  value  of  turbidity  readings,  expressed  in  terms  of 
the  reciprocal  scale,  has  not  been  appreciated.  Parmelee  and  Ellms, 
as  well  as  Hazen,  found  that  the  ratio  between  the  weight  of  sus- 
pended matter  and  turbidity  increased  with  the  turbidity,  but  the 
reason  for  this  was  attributed  to  the  differences  in  the  size  of  the  sus- 
pended particles,  — larger  particles  as  a rule  being  present  in  greater 
amounts  in  the  water  of  any  stream  during  periods  when  the  turbidi- 
ties are  high.  If,  however,  it  is  true  that  turbid  waters  diluted  one- 
half  do  not  have  half  the  turbidity  according  to  the  reciprocal  scale, 
it  follows  that  the  present  method  of  stating  results  is  not  the  best 
one,  as  the  figures  that  express  turbidity  do  not  give  true  comparisons 
of  the  amount  of  suspended  matter  present.  The  size  of  the  particles 
does  have,  indeed,  an  important  effect  upon  the  turbidity,  and  a given 
weight  of  fine  clay  particles  does  produce  a greater  opacity  in  the 

1 The  assumption  is  more  nearly  correct,  however,  for  observations  made  with  the  eye  at  a 
distance  of  five  or  six  feet  from  the  wire,  as  in  Hazen’s  practice,  and  this  fact  should  be 
remembered  in  the  present  discussion. 


28o 


George  C.  Whipple  and  Daniel  D.  Jackson. 


water  than  the  same  weight  of  coarse  silt  particles  ; but  the  effect 
of  the  water  itself  also  has  an  important  influence.  Th^  amount  of 
absorption  of  light  by  water  is  greater  than  is  sometimes  supposed. 
Wild  has  given  the  following  coefficients  of  absorption  for  distilled 
water : 


Temperature. 

Intensity  of  light  after  passing  through  io  cm. 
of  distilled  water. 

24.4°  C 

0.9179 

17.0 

09397 

6.2 

0.9477 

Not  only  does  the  absorption  vary  with  the  temperature,  it  varies 
greatly  with  the  character  of  the  dissolved  substances.  In  colored 
waters  the  absorption  is  much  greater  than  in  clear  waters,  but  the 
coefficient  is  at  present  unknown.  In  clear  waters  the  intensity  of 
the  light  at  various  depths  may  be  stated  approximately  as  follows : 


Depth  in  inches. 

Intensity  of  light. 

0 

1.00 

1 

.98 

5 

.92 

10 

.85 

15 

.79 

20 

.72 

It  is  apparent,  therefore,  that  the  absorption  of  light  by  the  water 
exerts  an  important  influence  on  the  visibility  of  the  wire.  The 
absorption  of  light  by  the  particles  of  suspended  matter  at  different 
depths  is  an  even  more  important  factor,  but  its  amount  is  unknown. 
Furthermore,  the  opaque  particles  act  as  a screen  to  shut  out  the 
object  in  view,  and  the  distance  of  these  particles  from  the  eye  must 
be  taken  into  account.  The  optical  phenomena  involved  are  compli- 
cated, and  it  is  unnecessary  to  analyze  their  relative  importance. 
It  is  sufficient  to  determine  at  what  depths  the  wire  becomes  invisible 
in  waters  of  definite  turbidity  with  different  degrees  of  dilution,  and 
from  these  to  formulate  a law  for  practical  application. 


Methods  Used  for  Measurement  of  Turbidity  of  Water. 


281 


Distilled  water  rendered  turbid  by  the  addition  of  1,000  parts  per 
million  of  finely  divided  silica  was  put  into  a jar,  and  the  depth  of  the 
disappearing  wire  observed.  It  was  then  diluted  with  distilled  water 
to  various  degrees,  and  a series  of  similar  observations  made.  The 
results  of  these  observations  are  shown  in  Figure  1. 

o 


/ 

OO  2 

1 1 1 | 1 

F>FF?TS  Psrej  M/LUO/V  OF  S/L/CF 

00  300  400  500  600  700  8< 

OO  9 

00  /c 

)00 

1 — 

tr=r=*-~ 

*--«* — 

\ 

'X  A 

y 

wS- 

£ 

Jr  7 r 

/o' 

d 

. ft 

I / 
X 1 

7 / 

/ 1 

•i 

DIAGRAM  SHOWING  THE  DEPTH 
AT  WHICH  A PL  A T/NU/Vt  WIRE  WILL 
BECOME  / HV/S/BLE  IN  WA  TERS 
MADE  TURBID  BY  DIFFEREN  T 
AMOUNTS  OF  SILICA. 

s 

- \ >o 
\ 

$ 

- ^ 
//? 

/ 1 
/ l 
/ l 
I l 
/ 1 
1 1 

fri — 

1 l i 

| 1 

L 1 
f f 

1 

/o 

« 

to 

on 

CU 

Fig.  1. 


It  was  found  that  when  the  water  contained  666  parts  of  silica  per 
million,  the  wire  disappeared  at  a depth  of  one  inch.  This  turbidity 
therefore  corresponds  to  unity  of  the  present  method  of  stating  results 
by  the  use  of  the  wire.  Starting  from  this  point,  the  curve  of  the 
reciprocal  scale  is  shown  by  the  dotted  line.  According  to  this  curve 
the  wire  should  have  disappeared  at  two  inches  when  the  water  con- 


282  George  C.  Whipple  and  Daniel  D.  Jackson. 

tained  333  parts  of  silica,  at  three  inches  when  it  contained  222  parts, 
and  so  on.  The  diagram  shows,  however,  that  the  depths  of  the 
disappearing  wire  did  not  follow  the  reciprocal  curve,  but  did  follow 
a somewhat  similar  curve.  Within  the  limits  of  practical  observa- 
tion this  curve  was  found  to  have  the  following  formula  : 

5 _ 400 

d — 0.4 

where  5 represents  the  number  of  parts  per  million  of  the  silica 
standard,  and  d represents  the  depth  of  the  wire  in  inches, 
or 

^ 1. 016 

==dr—  1.016 

where  d'  represents  the  depth  of  the  wire  in  centimetres, 
or 

£ 400  t 

1 — 0.4  t 

where  t represents  the  turbidity  expressed  in  terms  of  the  reciprocal 
scale. 

This  curve  is  shown  on  the  diagram  by  the  full  line. 

Observations  were  then  made  with  other  substances  than  silica,  to 
see  if  this  formula  could  be  generally  applied.  Natural  turbid  waters 
from  various  sources  were  diluted  with  distilled  water  and  the  depth 
of  the  disappearing  wire  ascertained.  Some  of  these  results  are 
shown  in  Figure  2. 

As  in  Figure  1,  the  dotted  line  represents  the  curve  of  reciprocals, 
and  the  full  line  the  curve  obtained  by  plotting  the  above  formula. 
In  Figure  2,  however,  the  abscissae  represent  not  weights  of  silica,  but 
a uniform  scale  of  turbidity,  where  unity  is  the  turbidity  that  causes 
the  wire  to  disappear  at  the  depth  of  one  inch.  The  black  spots 
represent  the  observations  obtained  by  the  authors  using  material  that 
was  obtained  from  the  experimental  filters  at  Pittsburg,1  Pa.,  to  pro- 
duce the  turbidity.  It  will  be  seen  that  they  follow  the  curve  very 
closely.  The  circles  represent  observations  obtained  by  Parmelee  and 
Ellms  on  the  water  of  the  Ohio  River.  These  also  follow  the  general 
curve.  Observations  with  natural  waters  and  with  waters  rendered 
artificially  turbid  with  kaolin  gave  similar  results,  while  waters  ren- 


1 This  material  was  kindly  furnished  by  Mr.  Wm.  R.  Copeland,  Bacteriologist-in-charge. 


Methods  Used  for  Measurement  cf  Turbidity  of  Water . 283, 

dered  turbid  with  colored  substances,  such  as  coal-dust,  lamp-black, 
iron  oxide,  etc.,  gave  but  slightly  different  results.  In  no  case  did 
the  observations  follow  the  curve  of  reciprocals,  and  in  every  case  the 
curve  was  similar  to  that  obtained  for  silica. 

It  is  apparent,  therefore,  that  the  above  formula  may  be  given 
general  application  within  the  limits  of  the  ordinary  observations  of 


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Fig.  2. 


turbidity  with  the  wire  method.  By  its  use  turbidities  obtained  by 
the  wire  method  may  be  expressed  in  terms  of  a uniform  scale,  a 
decided  advantage  over  the  reciprocal  scale. 

This  position  is  further  strengthened  by  a comparison  of  the  tur- 
bidity readings  with  the  amounts  of  suspended  solids.  It  has  been 
found  that  the  ratio  between  suspended  solids  and  turbidity  expressed 
by  reciprocals  of  the  depth  of  the  disappearing  wire  is  far  from 
constant,  but  that  the  ratio  between  suspended  solids  and  turbidity 


284  George  C.  Whipple  and  Daniel  D.  Jackson. 

calculated  to  silica  from  the  wire  readings  is  nearly  a constant  for 
samples  of  water  from  any  particular  locality.  The  following  obser- 
vations of  suspended  solids  and  turbidity  by  the  wire  method,  obtained, 
by  Parmelee  and  Ellms  for  the  Ohio  River,  illustrate  this  fact. 


Weight  of  sus- 
pended solids. 
(Parts  per  million.) 

Turbidity  by  wire 
method. 

(Reciprocal  scale.) 

Turbidity  reduced 
to  silica  standard. 

Weight  divided  by 
turbidity,  accord- 
ing to  recipro- 
cal scale. 

Weight  divided  by  tur- 
bidity in  terms  of  silica 
standard. 

33 

0.17 

75 

195 

.44 

62 

0.30 

136 

198 

.45 

126 

0.49 

244 

252 

.51 

248 

0.86 

527 

275 

.47 

342 

1.02 

690 

335 

.51 

453 

1.14 

833 

344 

.54 

528 

1.43 

1,333 

370 

.40 

Average,  .47 

Hazen,  in  his  Filtration  of  Public  Water  Supplies,  gives  the  fol- 
lowing equivalents  of  turbidity  readings  in  weight  of  suspended  matter 
for  river  waters  with  particles  of  average  size.  If  these  turbidities  are 
reduced  to  silica  by  the  formula,  the  ratio  between  turbidity  and  sus- 
pended matter  becomes  practically  a constant  when  the  turbidity  by 
the  wire  method  is  less  than  1.00. 


Turbidity  by  wire 
method. 

(Reciprocal  scale.) 

Turbidity  reduced 
to  silica  standard. 

Weight  of  sus- 
pended solids. 
(Parts  per  million.) 

Weight  divided  by 
turbidity  accord- 
ing to  recipro- 
cal scale. 

Weight  divided  by  turbid- 
ity according  to  silica 
standard. 

0.05  • 

21 

13 

260 

.62 

0.10 

42 

26 

260 

.62 

0.20 

88 

55 

275 

.62 

0.30 

136 

85 

283 

.62 

0.40 

189 

116 

290 

.61 

0.50 

249 

150 

300 

.60 

1.00 

666 

360 

360 

.54 

Average,  .61 

Methods  Used  for  Measurement  of  Turbidity  of  Water.  28  5 

The  following  table  will  be  found  convenient  for  transforming  tur- 
bidities obtained  by  the  use  of  the  wire  at  the  distance  of  normal 
vision,  and  expressed  according  to  the  reciprocal  scale  into  parts 
per  million  of  silica. 


TABLE  FOR  TRANSFORMING  TURBIDITIES  OBTAINED  BY  USING  THE  WIRE  METHOD 
WITH  THE  RECIPROCAL  SCALE  INTO  TURBIDITIES  EXPRESSED  IN  PARTS  PER 
MILLION  OF  THE  SILICA  STANDARD. 


Wire 

.OO 

.OI 

.02 

•03 

.04 

•°5 

.06 

.07 

.08 

.09 

reading. 

Parts  of 

■ Silica  Per  Million. 

.00 

00 

4 

8 

12 

16 

21 

25 

29 

33 

37 

.10 

42 

46 

50 

54 

58 

63 

68 

73 

78 

83 

.20 

88 

92 

96 

101 

106 

111 

116 

121 

126 

131 

.30 

136 

141 

146 

153 

158 

! 163 

168 

173 

178 

183 

.40 

189 

195 

201 

207 

213 

219 

225 

231 

237 

243 

.50 

249 

256 

262 

268 

275 

281 

287 

295 

302 

309 

.60 

316 

323 

329 

336 

345 

! 351 

358 

366 

373 

381 

.70 

389 

397 

404 

412 

421 

429 

437 

445 

453 

460 

.80 

470 

478 

487 

497 

506 

515 

524 

534 

543 

552 

.90 

562 

572 

582 

592 

602 

612 

623 

635 

645 

656 

1.00 

666 

... 

... 

Wire 

.OO 

.IO 

.20 

•30 

.40 

•50 

1 

.60 

.70 

| 

.80 

.90 

reading. 

Parts  of 

Silica  Per  Million. 

1.00 

666 

786 

923 

1,084 

1,273 

1,407 

1,777 

2,127 

2,564 

3,174 

2.00 

4,000 

... 

... 

.... 

It  is  preferable,  however,  to  have  the  turbidity  rod  graduated  in 
terms  of  silica  as  well  as  in  reciprocals.  Such  a graduated  rod  is 
shown  in  Figure  3.  For  the  purpose  of  graduating  the  rod,  or  for 
transforming  turbidities  expressed  in  depths  of  the  wire,  the  following 
table  will  be  found  useful. 


286 


George  C.  Whipple  and  Daniel  D.  Jackson . 


Fig.  3. — Turbidity  Rod. 


Methods  Used  for  Measurement  of  Turbidity  of  Water . 287 


TABLE  SHOWING  THE  DEPTH  OF  WIRE  IN  INCHES  AND  CENTIMETERS  CORRE- 
SPONDING TO  TURBIDITIES  EXPRESSED  IN  TERMS  OF  THE  SILICA  STANDARD. 


Silica. 

Depth  of  wire. 

Silica. 

Depth  of  wire. 

Silica. 

Depth  of  wire. 

Inches. 

Centime- 

ters. 

Inches. 

Centime- 

ters. 

Inches. 

Centime- 

ters. 

0 

.... 

65 

6.54 

16.5 

240 

2 06 

5.2 

1 

400.40 

1017.0 

70 

6.09 

15.5 

250 

2 00 

5 1 

2 

200.40 

509.0 

75 

5.73 

14.5 

260 

1.93 

4.9 

3 

133.73 

339.5 

80 

5.40 

13.7 

270 

1.88 

4.8 

4 

100.40 

264.2 

85 

5.08 

13.0 

280 

1.83 

4.7 

5 

80.40 

223.4 

90 

4.84 

12.2 

290 

1.78 

4.6 

■6 

67.06 

170.3 

95 

4.60 

11  7 

300 

1.73 

4.5 

7 

57  58 

146  3 

100 

4.40 

11.2 

400 

1.40 

3.5 

8 

50.40 

138  2 

no 

4.04 

10.2 

450 

1.29 

3.3 

9 

44.84 

113  8 

120 

3.73 

94 

500 

1.20 

3.1 

10 

40.40 

1118 

130 

3.48 

9.0 

600 

1.06 

2 7 

15 

27.00 

68.6 

140 

3.25 

85 

666 

1.00 

2.5 

20 

20  40 

61.0 

150 

3.06 

7.9 

700 

96 

2.4 

25 

16  40 

417 

160 

2.90 

7.3 

800 

90 

23 

30 

13  73 

348 

170 

2.75 

7.0 

900 

84 

2.1 

35 

11  83 

30  0 

180 

2.62 

66 

1,000 

80 

2.0 

40 

10.40 

26.4 

190 

2.50 

63 

— 

45 

9 28 

23  6 

200 

2 40 

6.1 

— 

.50 

8.40 

213 

210 

2 30 

5.8 

— 

— 

55 

7.67 

19.5 

220 

2.23 

5.6 

— 

— 

60 

7 07 

17.9 

230 

2.13 

5.3 

... 

The  Diaphanometer  Method. 

Parmelee  and  Ellms,  in  their  use  of  the  diaphanometer,  found  that 
the  ratio  between  the  suspended  solids  and  the  turbidity  expressed  in 
terms  of  the  reciprocal  scale  was  not  a constant,  but  increased  with 
i.the  turbidity,  as  shown  in  the  following  table  : 


288 


George  C.  Whipple  and  Daniel  D.  Jackson. 


Weight  of  suspended  solids. 
(Parts  per  million.) 

Turbidity  by  diaphanometer,  using 
reciprocal  scale. 

W eight  divided  by  turbidity. 

33 

.07 

465 

62 

.13 

484 

126 

.21 

583 

248 

.38 

657 

342 

.46 

745 

453 

.55 

820 

528 

.62 

855 

Accordingly,  they  adopted  the  following  formula  for  transforming 
turbidities  into  weights  of  suspended  matter : 

Weight  in  parts  per  million  = 600  (Turbidity  — 0.0 1), 

where  the  turbidity  equals  the  reciprocal  of  the  depth  in  inches  of  the 
water  in  the  diaphanometer  tube.  On  account  of  the  nature  of  sus- 
pended matter  in  the  water  used  in  their  work  this  formula  is  neces- 
sarily only  of  local  application. 

Through  the  courtesy  of  Mr.  George  W.  Fuller  the  authors  have 
had  the  privilege  of  experimenting  with  the  diaphanometer  used  by 
Parmelee  and  Ellms  at  Cincinnati.  Turbidity  observations  have  been 
made  upon  waters  rendered  artificially  turbid  by  the  addition  of  silica 
standard  in  various  amounts.  The  results  are  shown  in  Figure  4.  It 
was  found  that,  with  a turbidity  produced  by  1,000  parts  of  silica  per 
million,  the  cross  of  light  in  the  diaphanometer  tube  disappeared  from 
view  at  a depth  of  1.96  inches.  With  this  as  a starting  point,  the 
reciprocal  scale  would  follow  the  curve  shown  by  the  dotted  line. 
The  observations,  however,  did  not  follow  this  curve.  The  curve  of 
observations  was  similar  to  that  obtained  with  the  wire  method,  but 
the  variations  from  the  reciprocal  curve  were  somewhat  greater  than 
was  the  case  with  the  wire  method.  The  curve  of  observations  was 
found  to  have  the  following  formula  : 

d — 1 

where  5 represents  the  number  of  parts  of  silica  per  million  and  d 
equals  the  depth  of  the  liquid  in  the  tube  in  inches, 

•or 


Methods  Used  for  Measurement  of  Turbidity  of  Water.  289 


c = 2>433 

d'  — 2.54 

where  d!  equals  the  depth  in  centimetres, 
or 

5 = 958  t 
1 — t 

where  t represents  the  turbidity  according  to  the  reciprocal  scale. 


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This  formula  is  shown  on  the  diagram  by  the  full  line,  and  the 
observations  obtained  by  using  a Wellsbach  burner  are  indicated  by 
the  black  spots.  The  circles  represent  the  results  obtained  by  using 
daylight  reflected  from  a white  surface  instead  of  gaslight.  They 


290 


George  C.  Whipple  and  Daniel  D.  Jackson. 


show  that  the  observations  vary  greatly  with  the  intensity  of  the 
light,  a fact  naturally  to  be  expected.  It  is  evident,  therefore,  that 
for  accurate  work  the  light  must  be  under  definite  control,  and  that 
for  any  given  intensity  of  light  a corresponding  formula  must  be  used. 
In  other  words,  every  diaphanometer  must  be  calibrated  for  the  exact 
condition  under  which  it  is  to  be  used.  When  so  calibrated  the 
results  may  be  expressed  in  terms  of  the  silica  standard  according  to 
a uniform  scale. 


Comparison  with  Standards. 

The  direct  use  of  silica  standards  of  comparison  is  limited  to 
waters  having  turbidities  of  less  than  100  parts  of  silica  per  million. 
For  more  turbid  waters  it  is  necessary  to  dilute  with  clear  water 
before  reading.  With  very  turbid  waters  the  multiplied  error  of  read- 
ing becomes  appreciable,  yet  it  probably  does  not  exceed  the  errors  of 
the  other  methods  for  waters  of  the  same  turbidity.  If  a series  of 
standards  is  prepared  in  bottles  and  tubes  differing  by  5 parts  per 
million  between  o and  30,  it  is  possible  to  match  turbidities  within 
about  2 parts  per  million  ; above  30,  with  standards  differing  by  10 
parts,  it  is  possible  to  match  within  2 to  5 parts,  the  more  turbid 
waters  being  more  difficult  to  match. 


Comparison  of  Turbidity  Readings  by  the  Various  Methods. 

There  remains  to  be  considered  the  agreement  between  turbidity 
readings  obtained  by  the  various  methods.  Theoretically  the  read- 
ings obtained  by  the  wire  method  and  the  diaphanometer  should,  when 
reduced  to  terms  of  silica  by  the  formulae  given,  agree  with  readings 
obtained  by  direct  comparison  with  silica  standards.  As  all  of  the 
methods,  however,  are  subject  ta  error,  some  differences  in  the  read- 
ings are  to  be  expected.  The  following  table  shows  the  results  of 
turbidity  observations  made  by  the  three  methods,  upon  waters  from 
various  parts  of  the  country.  These  contained  clay  which  in  some 
instances  was  slightly  discolored  by  iron  oxide  and  organic  matter : 


Methods  Used  for  Measurement  of  Turbidity  of  Water.  29 


TURBIDITY. 

(Expressed  in  Parts  Per  Million  of  Silica.) 


Sample  number. 

By  wire  method. 

By  diaphanometer. 

By  comparison  with  silica 
standards  in  tubes. 

1. 

250 

239 

220 

2. 

150 

162 

140 

3. 

75 

79 

77 

4. 

480 

490 

450 

5. 

58 

55 

55 

6. 

720 

740 

680 

7. 

1,020 

980 

950 

8. 

740 

780 

730 

From  such  comparative  observations,  only  a few  of  which  are  given 
in  the  table,  it  has  been  found  that  the  average  deviation  from  the 
mean  of  the  three  readings  is  less  than  5 per  cent.,  a result  which 
may  be  considered  as  quite  satisfactory.  When  waters  are  rendered 
turbid  by  particles  which  have  a marked  color  of  their  own,  the  com- 
parison between  the  three  methods  fails,  as  described  beyond. 

The  ratio  between  the  turbidity  expressed  in  silica  and  the  weight 
of  the  suspended  solids  varies  with  the  size  of  the  particles  present. 
This  is  shown  by  the  following  table: 

Ratio  between  the  weight  of  suspended 
matter  in  parts  per  million,  and  tur- 


bidity  according  to  silica  standard. 

River  waters  with  coarsest  sediments,  (from  Hazen)  . 

1.09 

River  waters  with  average  sediments,  (from  Hazen)  . . 

.61 

River  waters  with  finest  sediments,  (from  Hazen)  . . . 

.39 

Ohio  River  water,  (from  Parmelee  and  Ellms)  .... 

.47 

Ohio  River  water  after  sedimentation,  (from  Parmelee 
and  Ellms) 

.28 

Hudson  River  at  Albany 

.72 

Passaic  River  at  Little  Falls 

.65 

Delaware  River 

.62 

Lake,  Prospect  Park,  Brooklyn 

.71 

Allegheny  River  at  Pittsburg 

.48 

292 


George  C.  Whipple  and  Daniel  D.  Jackson. 


When,  however,  comparisons  between  turbidity  and  weight  of 
suspended  matter  are  made  for  any  particular  locality,  the  ratio  is 
surprisingly  constant,  as  already  pointed  out.  It  is  possible  that 
some  universal  method  of  transforming  turbidity  readings  to  weights 
of  suspended  matter  might  be  devised,  based  upon  turbidity  readings 
made  before  and  after  various  periods  of  subsidence  under  definite 
conditions. 

Measurement  of  Turbidity  Produced  by  Colored  Particles. 

The  effect  of  the  color  of  water  upon  turbidity' observations  with 
the  wire  method  is  slight  in  very  turbid  waters,  but  becomes  apparent 
when  the  depth  of  wire  increases.  One  thousand  parts  of  silica  per 
million  were  added  to  distilled  water,  and  to  a water  that  had  a color 
of  1.04  on  the  Platinum  Scale.  These  were  diluted  to  various  degrees 
and  the  turbidities  observed.  The  results,  reduced  to  silica  by  the 
formula  given  above,  were  as  follows  : 


Calculated  Turbidity  Expressed  in  Parts  Per  Million 
of  Silica. 

Distilled  water. 

Colored  water. 

Original 

1,000 

1,000 

Diluted  \ 

500 

500 

Diluted  l 

250 

220 

Diluted  \ 

125 

112 

Diluted  

63 

55 

Diluted  Y2 

42 

36 

When  the  disc  method  is  used  in  comparatively  clear  waters  the 
effect  of  color  is  considerable,  as  shown  by  the  following  observations  : 


Color  of  the  Water.  (Platinum  Scale.) 

Depth  of  Disappearing  Disc  in  inches. 

.16 

174 

.56 

130 

.69 

no 

Methods  Used  for  Measurement  of  Turbidity  of  Water.  293 

Most  turbid  waters  met  with  in  nature  have  a dissolved  color 
insufficient  to  seriously  interfere  with  the  results  of  the  turbidity 
observations.  Some  waters,  however,  are  rendered  turbid  by  parti- 
cles which  are  themselves  colored.  Certain  streams  in  Pennsylvania, 
for  example,  are  black  from  the  presence  of  coal-dust ; others  are  red 
from  the  presence  of  large  amounts  of  clay  colored  by  oxide  of  iron  ; 
while  waters  receiving  discharges  from  dye-houses  and  other  forms  of 
manufacturing  waste  may  be  variously  colored.  It  has  been  found 
that,  because  of  the  absorption  of  light  by  these  various  x substances, 
the  optical  methods  of  determining  turbidity  fail  to  give  true  results. 

For  example,  a water  that  had  a marked  red  color  gave  a turbidity 
of  420  when  compared  with  the  silica  standards  in  the  tubes,  while 
by  the  wire  method  the  turbidity  was  660,  and  by  the  diaphanometer, 
684.  A water  that  contained  lampblack  gave  250  by  direct  com- 
parison and  470  by  the  wire  method,  while  another  sample  gave  50a 
and  1,300  respectively. 

In  order  to  see  whether  the  direct  method  of  comparison  was 
correct  when  applied  to  turbidities  produced  by  particles  different 
in  color,  lampblack  was  ground  until  the  particles  were  the  same 
size  as  those  of  the  silica  standard,  as  nearly  as  could  be  ascertained 
by  microscopical  measurement.  Two  series  of  turbid  waters  were  pre- 
pared, one  with  black  particles  and  the  other  with  approximately  equal 
numbers  of  white  particles.  Comparison  of  the  two  series  gave  the 
following  results  : 


Parts  Per  Million. 


Lampblack  .... 

20 

30 

40 

50 

60 

70 

80 

90 

100 

Silica 

18 

25 

32 

43 

52 

66 

70 

78 

88 

Ratio 

Average  ratio,  0.87 

.90 

.83 

.80 

.86 

.84 

.87 

.87 

.85 

.88 

The  turbidities  thus  produced  agreed  therefore  within  1 3 per  cent. 
Similar  comparisons  made  with  the  wire  method  gave  the  following 
results  : 


294 


George  C.  Whipple  and  Daniel  D.  Jackson. 


Parts  Per  Million. 

Lampblack 

63 

125 

250 

Wire  method  reduced  to  silica  . . . 

88 

207 

470 

Ratio 

1.42 

1.65 

1.44 

These  results  differed  by  about  50  per  cent.  With  deeper  tur- 
bidities the  ratio  was  still  further  increased. 

j 

Conclusions. 

From  the  preceding  investigations  it  appears  that  the  use  of  the 
silica  standard  permits  the  turbidity  of  water  to  be  accurately  ex- 
pressed in  terms  of  a uniform  scale  ; that  with  waters  of  ordinary 
turbidity,  results  obtained  by  the  use  of  the  wire  and  the  diaphanom- 
eter  may  be  reduced  to  the  silica  standard  with  a considerable  degree 
of  exactness ; that  with  waters  rendered  turbid  by  colored  particles 
the  optical  methods  fail  to  give  correct  results ; that  the  ratio  be- 
tween turbidity  in  terms  of  silica  and  weight  of  suspended  matter 
varies  with  the  size  of  the  particles,  but  is  nearly  constant  for  any 
particular  locality. 


Remark.  — After  experimenting  with  various  forms  of  diatomaceous  earth  in  the  pre- 
paration of  silica  standards,  the  authors  have  found  that,  while  any  such  earth  may  be  used 
if  the  directions  given  in  their  previous  article  are  followed,  the  most  satisfactory  earth  for 
the  purpose  is  that  from  a deposit  at  Herkimer,  New  York.  This  deposit  is  quite  free  from 
sponge  spicules  and  from  foreign  matter,  rendering  the  preparation  of  the  turbidity  standard 
a simple  operation.  No  washing  with  acid  is  necessary,  and  after  a short  ignition  and  a thor- 
ough grinding,  the  material  is  ready  for  use.  This  diatomaceous  earth  may  be  obtained  from 
George  W.  Searles,  Herkimer,  New  York. 

The  authors  have  had  recently  the  opportunity  to  compare  the  stock  mixtures  of  silica 
turbidity  standards  made  by  several  different  observers,  and  it  has  been  gratifying  to  find 
that  in  almost  every  case  the  agreement  has  been  within  the  limits  of  reading.  In  no  case 
did  one  standard  differ  from  the  others  by  more  than  3 per  cent.  During  the  process  of 
grinding  the  particles  of  silica  show  a slight  tendency  to  mat  together.  These  mats  should 
be  removed  before  weighing  by  sifting  the  material  through  a 200-mesh  sieve. 


